I think you will be lucky if there are any cabins spare with it being the school holidays. At Easter on Destiny I overheard one lady who needed a cabin swap for her disabled mum and she was told that it was full and not possible. I would go straight to reception when you board; if you have a late flight then I think any spares will have gone. There are always people who are new to Thomsons who haven't realised that the beds are in L shape and want to swap! I'm sure you'll enjoy it anyway!

Perhaps you could contact who you booked with - maybe it wouldn't cost too much to upgrade before you go.
I am sure you will be glad you did once you are onboard.
Bon Voyage!